444 MOLLUSCA. 



and are convex towards the aperture. The lobes and saddles may be themselves 

 secondarily folded or denticulated (Ammonites'), or the lobes alone, the saddles 

 being round (Ceratites}. 



The Aptychus of the Ammonitidae is a calcareous or horny plate, which was 

 probably secreted by the hood and served as an operculum ; it may be a single 

 plate, or divided by a suture into two (Synaptychus). 



Fam. 1. Nautilidae. Shell straight, or coiled ; aperture simple. Septa 

 concave towards aperture ; suture simple ; siphon usually central, or internal. 

 From Cambrian to present day; attained their highest development in Devonian 

 and Silurian. Orthoceras and Nautilus alone persist beyond the Palaeozoic epoch. 

 Nautilus L., sole living genus, N. fompilius L. ; Orthoceras Breynius, shell 

 straight, lower Silurian to Lias. 



Fam. 2. Ammonitidae. Shell of various forms, straight to spiral or turretted. 

 Septa much folded, suture complex ; siphon external. Extinct ; Silurian to 

 Eocene. Goniatites De Haan ; Ceratites De Haan ; Ammonites Bruguiere. 



Order 2. DIBRANCHIATA.* 



Cephalopoda with two gills in the mantle-cavity. 



The appendages are four pairs of arms with suckers on their oral 

 faces; in the Decapoda there are, in addition, two long, prehensile, 

 tentacle-like arms with suckers at their extremities only, placed 

 between the third and fourth arms. 



The cephalic cartilage constitutes a complete investment for the 

 -central nerve-organs. The eyes are elaborate, and have refractive 

 media. The lobes of the funnel are fused. An ink-sac is generally 

 present. The ctenidia are two in number, as are the branchial 

 vessels, auricles, and kidneys. There are no osphradia. The kidneys 

 open internally into the pericardium, and the latter has no external 

 opening. The shell is in many forms completely absent; in the 

 rest it is internal, or partly internal, and never protective to the 

 visceral sac. 



Sub-order 1. DECAPODA. 



In addition to the eight arms there are two long tentacles between the third 

 and fourth pairs of arms (ventral). The suckers are stalked and provided with 

 a horny rim. The eyes are without a sphincter-like lid. The mantle bears two 

 lateral fins, and there is a well-developed apparatus for closing the mantle- 

 opening. An internal shell is present. The heart is contained in the coelom. 

 Nidamental glands are generally present. 



Fam. 1. Ommatostrephidae. Tentacular arms short and broad; suckers 

 with toothed ring. Ommatostrcphes Gray, sagittated calamary, is able to 



* Hancock, "On certain points in the Anatomy and Physiology of the 

 Dibranchiate Cephalopoda," Nat. Hist. Jteview, 1861. R. Owen, "Supple- 

 mentary Observations on the Anatomy of Spirilla Australis," Ann. Mag. Nat. 

 Hist. (3), 3, 1879. T. H. Huxley and P. Pelseneer, "Report on Spirilla," 

 Challenger Reports, Pt. 83 (bound in at the end of the second volume of the 

 "Summary of Results"). 



